Official Luthiers Forum!

Very interesting. Gallagher makes a fine guitar.

I'm confused about elasticity. He is saying he makes some boards thicker to give more elasticity. That makes no sense to me at all.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Yes, I noticed he muddled up strength, stiffness and probably a couple of other things as well, but he meant, well, I think we know the general idea.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

I looked up the definition of elasticity and it seemed to make sense.

Their approach seems a bit simpler than the Gore method. Can anyone speak to the advantages of one over the other, is the Gore approach more accurate? Does it need to be?

Thanks, M

More elasticity means you can stretch further without damage and returns to the original length. So increasing the thickness of a board will decrease the elasticity. To me that's just common sense, I have forgotten everything I ever thought I knew about mathematics. I think the weight method will tell me that as well without all the calculus. If you need more weight to make the top move than less elastic, correct?

I'm f&%&$%n confused.

_________________
Hutch

Get the heck off the couch and go build a guitar!!!!

Hutch wrote:
"More elasticity means you can stretch further without damage and returns to the original length."

For wood the elastic measure is it's ability of the wood to return to it's original shape, as I understand it.
When thicknessing tops one method has you rest one end of the top on the bench, hold the other end and push on the middle and feel the way the top springs back. If you go too thin the top will feel floppy rather than "springy". It can be surprising how quickly this can change - which is a draw back to this method.
For rough thicknessing a number of tops this can still work well - just be sure to leave the top a little stiffer than what you want the final result to be.
I think the more "technical" method is to thickness them all to the same rough dimension and then measure the properties by various methods.

I think of it as, a board with more elasticity, can be thinner. Less flexible boards need to be thicker so they don't become cardboard. But guitar tops have many different bracing systems that change everything! Not going too thin allows the bracing to be thinner/lighter. But I have a hard time believing that too thin automatically means junk, and too thick means dead. I think the bracing adjustments can cover some of that.

I could be wrong!

On an archtop, cello, violin the tap tone method is even more handy, because what you have is what you have. Stronger arches, stiffer wood, and you can go thinner, but the tap tone can be a guide. They often add weight to the mix, to try to arrive at a prescribed number.

Two of the cello plans that I have exhibit very warped archings, more so than violins. Obviously going thinner than prudent hasn't hurt the sound, people still love it 300 years later. I'm guessing that they got most of their warp right away, and took a set.

Again, I could be wrong.

_________________
Why be normal?

That's backwards. Less flexible boards (stiffer boards) can be made thinner than more flexible (less stiff boards), which is what guitar builders typically do. If you start with two boards at the same initial thickness and one is stiffer then the other, the stiffer one can be thinned more than the floppier one.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

I was surprised to learn that elasticity means the opposite of what I intuitively thought.

https://en.wikipedia.org/wiki/Elasticity_(physics)

While what he says is interesting, I don't believe he's entirely correct in every aspect of his presentation. But it's a good starting point...

I guess I never really thought about it. It is interesting that I never connected the dots that elastic and plastic are opposites.

_________________
Bryan Bear PMoMC

Take care of your feet, and your feet will take care of you.

I always look at things in a different way. I can't figure out how some nouns are not verbs. I say they're verbs, they convey action. They probably have something that makes no sense to me. If you look at Eastern Red Cedar, and Sitka. ERC won't bend as far without breaking, so it has to be thicker. WRC would be a far better choice.

Wouldn't Sitka have a higher E,long than ERC? Doesn't that make it more flexible? It can take more flexing before it breaks. Isn't that more flexible? I guess the word that I'm thinking of is resilient. It's a better word.

Like I said, I'm not normal. I'm thinking of resilient not stiff. All wood doesn't bend to a certain point and then break. Maybe some is like glass, and will get to a certain point,without much movement and then shatter. I have no idea. I know a thin piece of ukulele wood can bend an inch without breaking. That is pretty impressive.

I should have said,

The wood with the higher E,long whatever THAT means, can be made thinner.

Sorry.

Ken

_________________
Why be normal?

Yea, we're using the word flexible in different ways. When I say one board is more flexible than another, I just mean it deflects more for a given force than the other board does. I get your meaning that a board that bends without breaking is flexible and a board that breaks when bent is not flexible, but in my experience that's not how people use it when talking about the flexibility/stiffness of soundboards.

_________________
Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Ken Nagy wrote:
"But I have a hard time believing that too thin automatically means junk, and too thick means dead. I think the bracing adjustments can cover some of that.
I could be wrong!"

You're not wrong per se, but it will affect the sound you get out of it. Lutes and ouds have much thinner tops than steel string guitars. In some cases the tops of ouds are less than one millimeter thick. Lutes have quite a number of braces glued across the top to reinforce it.
Measuring and testing the properties of the wood can allow you to build a guitar with a reasonable expectation that it will be structurally sound and sound like a guitar. Still, I think there are some things that can't be easily measured and quantified in the luthier's art.

P.S. - ERC has a little more than half the elastic measure of Sitka. But it is almost 20% heavier. To make it move better you might want to make it thinner to decrease the mass. would that make it more "flexible" or less? The meaning of "flexible" is a little too flexible.

Ken Nagy asked:
"Wouldn't Sitka have a higher E,long than ERC?"

E-long, the Young's modulus along the grain, is a measure of elasticity: the force it takes to stretch or compress a given size sample by a certain amount. Since most of the restoring force is bent beam or plate comes from stretching and compressing the surfaces along the bend E pretty well predicts the stiffness at a certain thickness.

It's interesting that E-long for all softwoods tends to track with density in the same way, within reason. So far, for all of the softwood samples I've tested, the E-long value ends up within about 10% plus or minus of the value you'd predict based on density for 60% or so of the samples. That's not bad for a natural material. That includes a few samples of Eastern red cedar.

When you talk about breaking strength that's another, different measure; the 'modulus of rupture'. I don't know as much about it as I'd like to, but I suspect it has to do with aspects of the way the cells are bonded together in the wood, or, perhaps, cell size? Western red cedar and redwood both tend to have high Young's modulus across the grain, but won't put up with much stress in that direction without cracking, where spruce tends to be tougher. Braces run up against the modulus of rupture more commonly than tops do, and I avoid using things like WRC for braces for that reason.

For the most part we worry more about stiffness and mass in a top than anything else: for most woods if the structure is stiff enough it will be more than strong enough, in the sense that the string tension won't pull it apart right away. Usually it takes a few years (or decades) of 'cold creep' under the static load, or some other damage, to make a well designed and built top fail.

Each top was described as having a different frequency.
It seems a little suspicious to me that the node lines form where the boards are supported.
Suspicious in the sense that could this have an impact on your findings?
Would it make more sense to fully support the bottom perimeter or something like that?

Just thinking/typing aloud.

_________________
Wake up and smell the rosewood!

You have to support the boards at the node lines so as not too inhibit the anti nodes.

I noticed that too.
When I referred back to Benade I saw that the first (and lowest hertz) vibration mode for rectangular plates had nodes in approximately those locations, as Ed mentioned. If he was looking for the lowest peak frequency supporting them at those locations makes sense. It's possible the width of the supports - might - affect the outcome some, but the outcomes would still be relative to each other.
I'm not sure I agree with everything in his methodology and conclusions, but if it works for him...